In recent years, due to the introduction of regulations for a coefficient of restitution of (SLE rule) in which a golf club should not have any spring like effect (SLE), the types of titanium alloy materials used for golf club faces have changed significantly. Before the regulation for a coefficient of restitution was put into effect, β type titanium alloys having low Young's modulus and thus easily attaining high restitution performances, and also having high strength and excellent durability were mainly used. Of these β type titanium alloys, a Ti-15% V-3% Cr-3% Sn-3% Al alloy mainly referred to as a Ti-15-3 alloy, and alloys analogous thereto have mainly been employed since they also nave excellent workability. However, due to the introduction of the regulation for a coefficient of restitution, in order to satisfy the regulation by lowering a coefficient of restitution using the β type titanium alloy in which an increase in a coefficient of restitution is likely to be caused by low Young's modulus, there is nothing for it but to increase rigidity of a face surface by increasing a face sheet thickness. In this case, an increase in material cost is unavoidable in the case of making a trial of applying a β type titanium, alloy containing a large amount of expensive alloying elements such as V and Mo to a face material. Furthermore, in the case of using the β type titanium alloy as compared with the case of using a material, which has a Young's modulus higher than that of the β type titanium alloy and also has a specific strength equal to or higher than that of the β type titanium alloy, there is a need to increase the sheet thickness, so that the face of β type titanium alloy becomes heavy. Thus, it is difficult to increase the volume of a golf club head in which the β type titanium alloy is used for a face, leading to relatively small sweet spot when hitting a golf ball, thus causing a problem that it is difficult for users to use the club. For such a reason, the β type titanium alloy is not used mainly for a material for a golf club face any more.
On the contrary, an α+β type titanium alloy having a Young's modulus higher than that of the β type titanium alloy is becoming the mainstream as a material for a driver face. By use of the α+β type titanium alloy having a high Young's modulus, even if the face is made thin, the coefficient of restitution is less likely to increase, and thus the degree of freedom of sheet thickness, which meets the regulation for a coefficient of restitution, increases as compared with the case of using the β type titanium alloy. Due to having a specific gravity smaller than that of the β type titanium alloy, the α+β type titanium alloy can increase the volume of a club head even if it has the same mass as that of the β type titanium alloy. The α+β type titanium alloy also has a lot of merits such as low material cost because of lower contents of alloying elements which are expensive compared with the β type alloy. This α+β type titanium alloy is typically a Ti-6% Al-4% V. In addition to this, for example, Ti-5% Al-1% Fe, Ti-4.5% Al-3% V-2% Fe-2% Mo, Ti-4.5% Al-2% Mo-1.6% V-0.5% Fe-0.3% Si-0.03% C, Ti-6% Al-6% V-2% Sn, Ti-6% Al-2% Sn-4% Zr-6% Mo, Ti-8% Al-1% Mo-1% V, and Ti-6% Al-1% Fe alloys are used.
Use of these alloys enables satisfaction of the regulation for a coefficient of restitution even if the face is made thin as compared with the face made of the β type titanium alloy, and also control within an appropriate range of strength and ductility enables imparting durability required for a golf club face. In this case, it is desirable that a round bar product or the like capable of controlling restitution performances by changing a face shape and structure has a Young's modulus of 120 GPa or higher, a tensile strength of 800 MPa or higher, and a total elongation of 15% or higher. It is desirable that a thin sheet product with low degree of working during face forming has a Young's modulus of 135 GPa or higher, a tensile strength of 1000 MPa or higher, and a total elongation of 10% or higher in one direction in the sheet face. It is desirable that the Young's modulus satisfies the value mentioned above so as to meet the regulation for a coefficient of restitution, and the tensile strength and ductility satisfy the respective values mentioned above so as to obtain satisfactory durability. However, due to insufficient workability of these alloys, it was difficult to stably supply a material, which satisfies the regulation for restitution coefficient and is excellent in durability and is also capable of increasing the face volume with the reduction in thickness of the sheet, with high production yield at low production cost.
For example, a Ti-6% Al-4% V alloy as a most versatile α+β type alloy has sufficient strength and sufficient Young's modulus as a face material, and is already used largely as an alloy for a golf club face. However, this alloy had some problems, namely, it contains 6% of Al having solid, solution strengthening ability at high temperature and increasing deformation stress during hot rolling, and thus has unsatisfactory hot workability, and also contains 4% of V of an expensive β-phase stabilizing element, and thus results in comparatively high material cost.
Patent Literature 1 proposes an alloy having a high specific strength like a Ti-6% Al-4% V alloy and with low material cost. This is an α+β type alloy which aims at a high specific strength and low cost by replacing expensive elements having high specific gravities such as V and Mo, as β-phase stabilizing elements, by inexpensive Fe having high β-phase stabilizing ability, and adding a large amount of Al as an α-phase stabilizing element having a small specific gravity. However, this alloy has a problem that it is difficult to undergo hot working it because of a high Al content, of 5.5 to 7%. In particular, in order to reduce production cost of a face material, it is required to supply the material in the form of a sheet product which can be formed into a face shape only by light press forming and polishing processes. However, it is difficult to form the alloy material into a sheet product due to a high hot deformation stress thereof. In particular, this alloy has a problem that an appropriate hot rolling temperature thereof is in a narrow range and during hot rolling, significant, edge cracking occurs if the temperature becomes slightly lower than the range, thus leading to a significantly low production yield.
On the other hand, examples of an α+β type alloy with comparatively satisfactory workability include an alloy proposed in Patent Literature 2. This alloy is characterized by being capable of undergoing uni-directional cold rolling in the form of coil without problem. However, this alloy also has a problem such as limitation of hot working temperature since it contains 4.5% of Al which causes deterioration of hot workability, and a problem such as high material cost since it contains expensive β-phase stabilizing elements of Mo and V in amounts of 2.0% and 1.6%, respectively.